Electron bombardment ion source



April 26, 1966 KlKUO SANO ETAL 3,248,587

ELECTRON BOMBARDMENT ION SOURCE Filed April 7, 1964 FIG FIG.2

l 3,248,587 ELECTRGN BUMEARDMENT IQN SUURCE Iiiltuo Sane and HitoshiTsuyama, Kitatama-gun, Tokyoto, .lapan, assignors to Kahushiki KeishaHitachi Selsa= inisho, Tokyoto, Japan, a joint-stock company of JapanFiied Apr. '7, I964, Ser. N 357,879 Claims priority, application Japan,Apr. 18, 1963, SEE/28,262 3 Claims. (Cl. 313-85) This invention relatesto improvements in ion sources of the electron bombardment type.

It is a general object of the invention to overcome certain difficultiesencountered in the prior art and to improve the collimation of electronbeams used in ionization.

The nature, principle, and details of the invention, as well as specificobjects thereof, will be best understood by reference to the followingdescription, taken in conjunction with the accompanying drawing in whichlike parts are indicated by like references characters, and in which:

FIGURE 1 is a sectional view showing the essential parts of one exampleof an electron bombardment ion source;

FIGURE 2 is a perspective view showing the essential parts of oneexample of an ion source of known type; and

FIGURE 3 is a perspective view showing the essential parts of apreferred embodiment of the ion source according to the invention.

in mass spectrometers for gas analysis, ion sources of the electronbombardment type in which ionization is accomplished by bombarding gasmolecules with electrons are widely used. The ionization region in oneexample of this type of ion source is shown in FIGURE 1. An elec tronbeam e emitted from a ribbon or tape filament 2, focused by a grid 3,and accelerated by an anode 4- is caused to collide with gas molecules gintroduced into an ionization chamber 1, thereby to accomplishionization of the gas molecules g and cause an ion current to begenerated. An electron collector 5 is provided at the end of theionization chamber 1 opposite to the filament 2.

In this case, in order to generate a large quantity of ions, a filament2 in the form of a tape is used, but in the case wherein the collimationof the electron current is poor, the electron current passing throughthe entrance slit for electrons becomes small, and, moreover, an energyspread in the generated ions occurs, whereby a lowering of the accuracyof mass analysis occurs. For this reason, the collimation of a stableelectron current is required.

Heretofore, for this purpose, a grid electrode 13 as shown in FIGURE 2,which is a perspective view in the direction indicated by A in FIGURE 1,has been used. A tape filament 12 provided at its ends with filamentanchors 6 and e, is interposed centrally between the end edges of theflanges of a grid electrode 13, which is made of a metal plate formedinto a configuration with a channel cross section having rectangularprojections 31 at the central parts of the end edges of its flanges.When a current is passed through the filament 12 to cause it to emit anelectron beam, and negative and positive potentials are appliedrespectively to the grid electrode 13 and the anode 4, an electroncurrent is generated, on which there acts an electric field collimatingelectrons in the x direction, but in the y direction there acts anelectric field which diffuses electrons. For this reason, collimation inonly one direction is possible. Consequently, the re quired conditionsof constricting the electron current as much as possible and of causingas large a quantity of electrons as possible to pass through theentrance slit 4 for electrons are not fulfilled, and, accordingly, thecurrent passing through the entrance slit 4 is small for the samemagnitude of thermionic emission. Moreover, since the collimation ispoor, the electron current reaches a wide area, thereby soiling theinternal surface of the ion source chamber.

States Patent 0 It is a specific object of the present invention toovercome the above-described difficulties and to improve the collimationof electron beams used for ionization.

The nature and details of the invention will be more clearly apparentfrom the following description of a preferred embodiment of the electronbombardment ion source according to the invention as shown in FIGURE 3,which shows an electron gun suitable for use in the said embodiment andwhich is a perspective view similar to that in FIGURE 2. The electrongun shown in FIGURE 3 differs from that shown in FIGURE 2 in that cutoutrecesses are provided in the central parts of the projections 31 formedon the end edges of the flanges of the grid electrode 23. As a result,each of the projections 31 has substantially prominent projections 32and 33 at its two ends.

As the grid electrode, its construction may be such that it is providedwith mutually opposite sides between which the filament is disposed andit will be obvious that the cutout recesses may be provided on the endedges of the flanges of a grid electrode which has no projections.

By the above-described construction of the grid, collimation is effectednot only in the x direction but also in the y direction as shown inFIGURE 3. Consequently, the electron beam is subjected to collimationfrom four directions, and collimation is substantially improved. As aresult, the current used for ionization increases and, since dilfusionof the electron current is prevented, the soiling due to impingement ofelectrons on the source chamber wall is reduced. Moreover, the energyspread of the ions can be reduced. Therefore, all of these favorableresults are advantageous for increasing the accuracy of massspectrometry.

While the above-described embodiment of the invention has been describedwith respect to its use for a mass analyzer, it will be apparent the ionsource of the present invention is effectively applicable to many otherkinds of apparatuses and instruments.

It should be understood, of course, that the foregoing disclosurerelates to only a preferred embodiment of the invention and that it isintended to cover all changes and modifications of the example of theinvention herein chosen for the purposes of the disclosure, which do notconstitute departures from the spirit and scope of the invention as setforth in the appended claims.

What is claimed is:

1. In an ion source of the type wherein ionization is accomplished bymeans of an electron beam of sheet-like configuration emitted from anelongated filament, a pair of grid electrode plates having their flatsurfaces parallel to each other and to the longitudinal axis of saidfilament interposed therebetween, each of said electrode plates lying ina plane parallel to the direction of the electron beam emission andhaving at the central part of its edge in the direction of the electronbeam emission projections with a cutout recess therein, the resultingconstruction making possible tridimensional control of collimation ofsaid electron beam.

2. The ion source as defined in claim 1, wherein said filament is awire.

3. The ion source as defined in claim 1, wherein said filament is atape.

References Cited by the Examiner UNITED STATES PATENTS 2,820,169 1/1958Morris 313- X 3,062,984- 1l/1962' Hofker 313-69 X GEORG N. WESTBY,Primary Examiner.

ROBERT SEGAL, Examiner.

S. D. SCHLOSSER, Assistant Examiner.

1. IN AN ION SOURCE OF THE TYPE WHEREIN IONIZATION IS ACCOMPLISHED BYMEANS OF AN ELECTRON BEAM OF SHEET-LIKE CONFIGURATION EMITTED FROM ANELONGATED FILAMENT, A PAIR OF GRID ELECTRODE PLATES HAVING THEIR FLATSURFACES PARALLEL TO EACH OTHER AND TO THE LONGITUDINAL AXIS OF SAIDFILAMENT INTERPOSED THEREBETWEEN, EACH OF SAID ELECTRODE PLATES LYING INA PLANE PARALLEL TO THE DIRECTION OF THE ELECTRON BEAM EMISSION ANDHAVING AT THE CENTRAL PART OF ITS EDGE IN THE DIRECTION OF THE ELECTRONBEAM EMISSION PROJECTIONS WITH A CUTOUT RECESS THEREIN, THE RESULTINGCONSTRUCTION MAKING POSSIBLE TRIDIMENSIONAL CONTROL OF COLLIMATION OFSAID ELECTRON BEAM.